Conventional golf balls can be divided into three general types or groups: (1) two piece balls, (2) wound balls (also known as three piece balls) and (3) multilayer balls. The difference in play characteristics resulting from these different types of constructions can be quite significant.
Balls having a two piece construction are generally most popular with the average recreational golfer because they are relatively durable while also providing maximum distance. Two piece balls are made with a single solid core, usually formed of a crosslinked rubber, which is encased by a cover material. Typically the solid core is composed of polybutadiene which is chemically crosslinked with zinc diacrylate and/or similar crosslinking agents and is covered by a cover material such as an ionomer resin (e.g., SURLYN.RTM. (DuPont) and Iotek.RTM. (Exxon)) or blends thereof, polyurethane or balata (transpolyisoprene rubber).
Wound balls typically have either a solid rubber or liquid center core around which many yards of a stretched elastic thread or yarn are wound. The wound core is then covered with a durable cover material such as an ionomer resin or a softer material such as balata or polyurethane. Wound balls are generally softer and provide more spin than two piece balls, which enables a skilled golfer to exercise greater control over the ball's flight and final position.
Relatively recently, a number of golf ball manufacturers have introduced multilayer golf balls in an effort to overcome some of the undesirable aspects of conventional two-piece balls such as their hard feel, while maintaining the positive attributes of these balls such as their increased initial velocity and distance. Further, it is desirable that such multilayer balls have a "click" and feel as well as spin characteristics approaching that of wound balls.
Multilayer golf balls can be formed using a variety of constructions. For example, multilayer balls may have two or more cover layers molded around a conventional core. Alternatively, they may comprise a conventional cover and a core with one or more intermediate layers interposed between the cover and the core. Likewise, multilayer balls may be formed from cores having more than one core layer and may optionally contain one or more intermediate and/or cover layers. Multilayer balls may even comprise a conventional wound core around which at least one intermediate layer and/or at least one cover layer is formed. Examples of multilayer balls that are presently commercially available include the Episode (Titleist), Altus Newing (Bridgestone), Giga (Spalding), Metal Mix (Dunlop) and Ultra Tour Balata (Wilson).
Typically, the layers of multilayer golf balls are formed by molding them around the core or a preceding intermediate layer or cover layer. Conventional techniques for applying such layers include injection molding, compression molding and casting the layer material around the preceding core or layer. Accordingly, a crucial aspect of the manufacture of multilayer balls is obtaining good adhesion between the various layers. If the adhesion between the layers does not meet desired standards, the performance of the golf ball will be adversely affected. For example, poor adhesion can cause air pockets between the layers which can result in separation of the layers when the ball is struck with a club.
It is well known that the adhesion between the wound core and the cover of a wound ball is enhanced due to the small imperfections created in the uneven outer surface of the winding formed by overlapping the thread. Cover material flows into these imperfections when the cover is molded about the wound core, resulting in improved adhesion. In contrast, however, in two piece and multilayer balls, adhesion between the core, cover and/or intermediate layers is greatly reduced due to the smooth outer surface of the layers.
Accordingly, there are a number of methods known in the art directed towards promoting adhesion between the various solid layers of a golf ball. For example, U.S. Pat. Nos. 4,229,401 and 4,173,345 are directed towards alleviating problems associated with compression molding covers about a core by providing a series of surface channels 1/16 of an inch deep which encircle the outer surface of the core and pass through both polar areas thereof. Providing such channels was found to prevent separation of the cover from the core when the ball is struck with a golf club by eliminating the entrapment of air between the cover and the core during the compression molding process.
Additionally, there are a number of patents directed towards promoting adhesion between core layers and covers, as well as adhesion between intermediate layers, including wound layers, through the use of projections or prongs extending from the outer surface of the core or an intermediate layer. For example, U.S. Pat. Nos. 720,852 and 2,229,170 disclose the use of a plurality of uniform sized projections extending from the surface of the core. Such projections are designed to provide an anchoring means for an elastic thread winding.
Moreover, U.S. Pat. No. 697,925 discloses game balls wherein a metallic core is covered by a soft rubber envelope which contains a plurality of deep "pits" extending about halfway through the envelope. A hard shell material is compression molded around the core/envelope such that the shell material flows into the pits, thereby forming prongs which interlock the hard shell with the soft rubber envelope.
Additionally, it is known in the golf ball art that adhesion between the cover and the core of a two-piece golf ball can be improved by roughening the outer surface of a core layer. For example, U.S. Pat. No. 1,558,706 provides a very general disclosure of improving the adhesion of a two piece ball having a vulcanized rubber core and a balata shell, wherein the outer surface of the core is roughened by forming corrugations which assist in interlocking the cover and the core so as to prevent any movement therebetween when the ball is struck with a club. Although the '706 patent discloses the general concept of roughening the core surface to improve adhesion, it does not provide any details as to how the core surface is roughened nor any specifics regarding the characteristics of the corrugations in the core surface.
Similarly, U.S. Pat. No. 4,367,873 discloses making a softball or a baseball having a PVC foam core and an ethylene copolymer core cover, wherein the core cover has corrugations one eighth to one sixteenth of an inch high. A layer of yarn windings is wound around the core cover and a leather outer cover is then placed around the windings. The '873 patent discloses that such corrugations may have a buffering effect when the ball is impacted and provide a more resilient ball as well as possibly increasing the binding friction between the outer leather cover and the inner core.
One conventional method for improving the adhesion between the layers of a multilayer golf ball is to texture the outer surface of, e.g., a core or a core with an intermediate layer(s) molded around it by manually roughening or scuffing the outer surface after molding and prior to forming the subsequent layer thereon. Such surface roughening is typically obtained by grinding the surface with a grinder such as a Glebar grinder or by tumbling or milling the ball at various stages of its construction in an abrasive media. Likewise, the surface may be roughened by blasting the surface of the layer with sand or some other abrasive material.
After the surface is roughened, it must be washed to remove any loose material or excess abrasive material. otherwise, the loose material and/or excess abrasive material which remains on the surface will form air pockets when the next layer is molded thereon. As mentioned above, such air pockets are undesirable as they lead to separation of the layers. Further, the roughened surface must be completely dried before the next layer can be applied. If it is not dried completely, the remaining moisture is converted into steam when the next layer is molded around it, forming defects in the interfacial bonding area that can also lead to separation of the layers.
Although such post-molding methods roughen the outer surface of the layer, it is difficult to obtain sufficient texture depths with these processes. Moreover, even after washing and drying the treated surface, there is a tendency for small particles from the golf ball layers and/or excess abrasive media to remain in the interfacial bonding area between the layers which are easily dislodged when the ball is struck with a club and can form air pockets between the layers which can lead to the separation of the layers. Similarly, inadequate texture depth can also lead to separation of the layers.
Further, such post-molding roughening methods require, at a minimum, three distinct steps: 1) roughening the surface via grinding; 2) washing the roughened surface to remove excess debris; and 3) thoroughly drying the roughened surface before the next layer can be applied. Each of these process steps are labor intensive, time-consuming and add additional expense to the production of multilayer golf balls.
Thus, there remains a need for an efficient method for improving the interfacial adhesion between golf ball layers, wherein the outer surface of a layer is provided with a texture of an adequate depth and free of debris, while eliminating any post-molding treatments such as grinding, sandblasting, washing and drying.